The present invention is concerned in general terms with diaphragm clutches, especially for motor vehicles, and is directed more particularly to that part of these clutches, commonly referred to as the clutch mechanism, which, constituting by itself a unitary assembly, is to be fitted as such on a plate, referred to as a reaction plate, after a friction plate has been inserted, so as to constitute such a clutch.
In general terms, and as described in the document FR-A-2 463 874, such a mechanism comprises a hollow cover plate having at its outer periphery an annular skirt, oriented generally axially, at least one pressure plate, which is coupled in rotation to the cover plate through attachment means which enable the said pressure plate to be displaced axially with respect to the said cover plate, and, between the cover plate and the pressure plate, a diaphragm the outer periphery of which defines a Belleville ring, which, firstly, bears at its outer periphery on a deflection abutment carried by the cover plate, and, secondly, bears at its inner periphery on a projecting element of the pressure plate so as to urge the said plate in a direction in which it is displaced away from the cover plate.
Such a mechanism poses problems. In this connection, having regard to manufacturing tolerances, the diameter of the peripheral skirt of the cover plate may vary not insignificantly, for example in the order of 1 mm for a cover plate size having a diameter of the order of 220 mm.
The same is true, to a lesser extent, for the diaphragm, since its manufacturing tolerances are of course more strict, but it is subjected to ovality effects due in particular to the heat treatment to which it is subjected so as to give it a certain hardness and elasticity. For example, the diameter of the diaphragm may vary by 0.5 mm for a cover plate size with a diameter of 220 mm.
Conventionally, the mechanism is dynamically balanced before leaving the production line, but once it is fitted in the vehicle, this mechanism is no longer able to be dynamically balanced, the diaphragm being displaced with respect to the cover plate under the effect of parasitic movements such as shocks, wear in the deflection abutment, longitudinal and radial movements of the crankshaft of the engine, etc.